1. Field of the Invention
The present invention relates to a layered crystal structure oxide such as a so-called Aurivillius crystallographic group and a superconducting material and a process for producing the same.
2. Description of the Related Art
Bismuth layered crystal structure oxides form an industrially very important compound group such as bismuth superconducting oxides having a critical temperature of 110.degree. K and bismuth layered ferroelectric materials which are recently spotlighted as materials for ferroelectric non-volatile memories (FeRAM) (C. A. Paz de Araujo, J. D. Cuchiaro, L. D. McMillan, M. C. Scott and J. F. Scott, Nature, 374 (1995) 627.; K. Amanuma, T. Hase and Y. Miyasaka, Appl. Phys. Lett., 66 (1995) 221.; S. B. Desu and D. P. Vijay, Master. Sci. and Eng., B32 (1995) 75 and the like). In particular, bismuth.strontium.tantalate: Bi.sub.2 SrTa.sub.2 O.sub.9 (hereinafter called BiSTa) is shown to be excellent in fatigue endurance following polarization inversion and capable of driving at a low voltage and is considered to be the most promising candidate for a capacitor material for FeRAM (C. A. Paz de Araujo, J. D. Cuchiaro, M. C. Scott and L. D. McMillan, International Patent Publication No. WO 93/12542 (Jun. 24, 1993)).
In recent years, it is reported that BiSTa has successfully been prepared by an MOCVD process (metal organic chemical vapor deposition) (T. Ami, K. Hironaka, C. Isobe, N. Nagel, M. Sugiyama, Y. Ikeda, K. Watanabe, A. Machida, K. Miura and M. Tanaka, Mater. Res. Soc. Symp. Proc., 415 (1996) 195.; T. Li, Y. Zhu, S. B. Desu, C, H. Peng and M. Nagata, Appl. Phys. Lett., 68 (1996) 616.).
Meanwhile, a number of arguments on the relation of a composition of BiSTa with a ferroelectricity thereof have so far been made, and it is reported that the most excellent ferroelectricity can be obtained in a composition in which bismuth is a little excessive and strontium is slightly deficient, typically in the vicinity of Bi.sub.2.2 Sr.sub.0.8 Ta.sub.2 O.sub.9 (T. Noguchi, T. Hase and Y. Miyasaka, Jpn. J. Appl. Phys., 34, 4900 (1996) and the like). Thus, it is presumed that the partial substitution of bismuth with strontium may be caused in this optimum composition.
However, a great part of these arguments on the compositions is made based on the compositions of the starting materials. Or, also in different cases, the arguments are made based on a polycrystal thin film in which a by-produced phase can be present in a grain boundary and others, and therefore a problem is present in terms of data accuracy. It is reported, for example, that while a single phase has been observed by X-ray analysis, the presence of an amorphous phase has been confirmed by observation with a high resolution transmission electron microscope (TEM) (C. D. Gutleben, Y. Ikeda, C. Isobe, A. Machida, T. Ami, K. Hironaka and E. Morita, Mater. Res. Soc. Symp. Proc., 415 (1996) 201.). Accordingly, it is completely indistinct as well if the partial substitution of the elements is caused.
Further, considering this from the viewpoint of a crystal structure, this BiSTa is called a so-called Aurivillius crystallographic group, and the fundamental crystal structure (host structure; space group I4/mmm or F4/mmm) of Bi.sub.2 SrTa.sub.2 O.sub.9 is as shown in FIG. 1. In this connection, the Aurivillius crystallographic group is represented by the composition formula of [Bi.sub.2 O.sub.2 ].sup.2+ [Me.sub.m-1 R.sub.m O.sub.3m+1 ].sup.2-, wherein m is an integer of 2 or more; Me is at least one selected from the group consisting of sodium (Na), potassium (K), calcium (Ca), barium (Ba), strontium (Sr), lead (Pb) and bismuth (Bi); and R is at least one selected from the group consisting of iron (Fe), titanium (Ti), vanadium (V), niobium (Nb), tantalum (Ta) and tungsten (W).
As shown in FIG. 1, BiSTa can be considered to have a structure in which a layer (perovskite layer) comprising two unit lattices of a perovskite structure (ABO.sub.3) where strontium is an A site and tantalum is a B site is interposed between layers (fluorite layers) of a fluorite structure composed of bismuth and oxygen. In general, it is considered that out of the above layers, the perovskite layer is responsible for ferroelectricity, and the fluorite layer has a function to maintain the crystal structure. Incidentally, bismuth of the A site in the boundary of the perovskite layer is shared with the fluorite layer.
However, researches on the production process by the MOCVD method have made it clear that this substance has a large non-stoichiometric property and can be formed by using a metastable fluorite phase shown in FIG. 25 as a crystal nucleus and subjecting this to heat treatment (T. Ami, K. Hironaka, C. Isobe, N. Nagel, M. Sugiyama, Y. Ikeda, K. Watanabe, A. Machida, K. Miura and M. Tanaka, Mater. Res. Soc. Symp. Proc., 415 (1996) 195.). Further, it has been confirmed in different researches that also when the crystallinity of a fluorite phase is reduced by lowering the temperatures in the producing process to form the precursor thereof rather close to amorphous one, the layered crystal structure described above is formed through this fluorite phase in a subsequent heat treatment process (C. Isobe et al., 9th Int'l Symp. on Integrated Ferroelectrics Abstr. & Program, 79i). In this case, only one metal site is present in the fluorite phase, and therefore all metal elements of bismuth, strontium and tantalum are considered to occupy an equivalent site over the broad composition range.
In the fluorite phase, a considerably large amount of alkaline earth metals and others can be added to a zirconium (Zr) site in partially stabilized zirconia to form a solid solution. Further, as can be seen from the fact that an oxygen excess type non-stoichiometric compound can be formed in uranium oxide, a non-stoichiometric property is present. As a matter of course, the presence of such non-stoichiometric property applies to deficiency, and both cations and anions can have a large deficiency density. In fact, eight anions have to ideally coordinate around a cation in this fluorite structure, but there is an example in which a coordination number of oxygen is decreased to about 6, by oxygen deficiency while maintaining this structure, as is the case with .delta.-Bi.sub.2 O.sub.3 (ICPDS, 16-654; .delta.-Bi.sub.2 O.sub.3 ; Gattow and Schroder, Z. Anorg. Allgem. Chem. 318 (1962), 176). In this case, out of oxygen atoms forming a regular hexahedron by 8 coordination, two oxygen atoms disposed diagonally to each other are selectively deficient, and the remaining oxygen atoms form a distorted octahedron around bismuth. Because of such property, the fluorite phase is tolerant to lattice distortion and uneven distortion in crystal. Typically, the symmetrical property is lowered in a regular tetrahedron through monoclinic system at room temperatures by lattice distortion in zirconia, and oxygens take 7 coordination. It is not rare that a half band width of X-ray diffraction peaks by .theta.-2.theta. scanning comes up to several degrees or more by uneven distortion.
On the other hand, some non-stoichiometric property is present, though not so much as the fluorite phase, as well in the perovskite phase shown in FIG. 26. Examples of the non-stoichiometric property at the A site include various ones such as oxide superconducting materials, for example, those obtained by adding barium, strontium and potassium to a lanthanum site of La.sub.2 CuO.sub.4. Examples of the non-stoichiometric property at the B site include those obtained by adding lead to a bismuth site of BaBiO.sub.3.
Thus, from the viewpoint of a forming mechanism, this material can be defined by .left brkt-top.structure in which metal atoms are regularized from the metastable fluorite phase having a large non-stoichiometric property by heat treatment.right brkt-bot.. However, considering that a part of the fluorite structure is left as well in the structure itself and a non-stoichiometric property is present in a perovskite portion to some extent, it is difficult to expect that this regularization goes on always in a perfect manner. It is so much the more when a deviation in the composition is present.
Accordingly, single crystal as a sample is very important for discussing such composition and crystal chemical side view. With respect to single crystal in the Aurivillius crystallographic group, there have been made researches on Bi.sub.4 Ti.sub.3 O.sub.12 (m=3; Me=Bi; R=Ti) (J. F. Dorian, R. E. Newnham, D. K. Smith and M. I. Kay, Ferroelectrics, 3 (1971) 17 and the like) and in recent years, on Bi.sub.4 BaTi.sub.4 O.sub.15 (m=4; Me=Bi; R=Ti) (S. K. Kim, M. Miyayama and H. Yanagida, J. Ceram. Soc. Japan, 102 (1994) 722 and the like). However, reliable report examples on BiSTa are very few (R. E. Newnham, R. W. Olfe, R. S. Horsey, F. A. Diaz-Colon and M. I. Kay, Mater. Res. Bull., 8 (1973) 1183.; A. D. Rae, J. G. Thompson and R. L. Withers, Acta. Cryst., B48 (1992) 418.). In addition, out of two papers on BiSTa, the paper reported by Newnham is lacking in accuracy in handling the compositions. Meanwhile, in the paper reported by Rae, tabular single crystal is obtained only in a two phase-mixing state while starting in a fixed ratio composition, and a single phase has not come to be synthesized. Further, the characteristics of the resulting single crystals have scarcely been analyzed in both papers.
That is, it has very important meaning in applying the single crystal of the Aurivillius crystallographic group to FeRAM as a material showing ferroelectricity to prepare the single crystal of the Aurivillius crystallographic group and make a crystal chemical side view thereof clear. Further, it is indicated as well that these Aurivillius crystallographic groups have possibility to show paraelectricity depending on the composition and the crystallinity thereof (M. Machida, N. Nagasawa, T. Ami and M. Suzuki, Applied Physics Society [9p-F-2], No. 57, Autumn 1996), and they have a very important meaning as well in using them as a new DRAM material and applying them to a new laminated device.
It is proposed to apply the self-flux method which is used in preparing high temperature superconducting oxides (refer to Y. Hidaka, Y. Enomoto, M. Suzuki, M. Oda and T. Murakami, J. Cryst. Growth, 85 (1987) 581; and Y. Hidaka, M. Oda, M. Suzuki, Y. maeda, Y. Enomoto and T. Murakami, Jpn. J. Appl. Phys., 27 (1988) L538.) to the preparation of the single crystal of the Aurivillius crystallographic group (M. Suzuki, N. Nagasawa, A. Machida and T. Ami, Jpn. J. Appl. Phys., 35 (1996) L564.). In this connection, the flux method is a method in which a suitable amount of a flux (fusing agent) is added to a raw material to prepare a fused liquid by heating, and then this is cooled down to generate a crystal nucleus to grow crystal. In this flux method, the flux usually remains even after growing the crystal, and the grown crystal has to be separated from the flux to take it out.
However, there have been involved in preparing the Aurivillius crystallographic group by the self-flux method, the problems that when trying to remove the flux by bleaching, it can not be removed because the flux is not dissolved in water and that when trying to remove the flux by etching with acid such as hydrochloric acid (HCl), crystal grown together with the flux is eroded as well, and therefore it is difficult to take out the crystal.